Prior-art fault localizing and identifying devices of the type mentioned above are used to localize, for instance, short circuits or earth faults or to follow a conductor through the system. It should be mentioned that the term electric system includes, inter alia, systems of transmission lines, fire alarm systems and house electric mains. The known devices are made up of two units having a transducer/detector or transceiver configuration. As a rule, one part is connected to a line at one end thereof, for instance, in a switch cabinet, whereupon the actual fault localizing or identification is carried out by means of the other unit at a distance from the first unit. The distance can be considerable and may, in some cases, amount to ten, twenty or more kilometers.
A known device is disclosed in DE-44 29 310. This device is used to localize earth faults in three-phase networks. It comprises a primary unit in the form of a pulse generator emitting pulses on the phase conductors via an earth coil and a secondary unit in the form of a detector detecting induced current changes at the beginning of the pulses. The change is an increase if the conductor portion between the coil and the detector is correct, but a decrease if the earth fault is located along said conductor portion.
Another prior-art device is disclosed in U.S. Pat. No. 4,642,556. This device is used to identify conductors in a power distribution network. It comprises a primary unit in the form of a transmitter which transmits current pulses in the conductor that is to be identified in the network and a detector which inductively detects the electromagnetic field generated by the current pulses around the conductor.
There are also known devices comprising a primary unit which is connected to one end of a line and which comprises resistors with different values, which are connected to the conductors, and a secondary unit which is connected to the other end of the line and which detects the resistance in the various conductors as well as the resistances in the primary unit.
A common feature of the known devices is that they are usable to a limited extent, either for a specific task, such as the two tasks first described, or for a small number of tasks, such as the last-mentioned type. They do not manage a combination of, for instance, detecting earth faults and short circuit faults in a pure main circuit and identifying a conductor in a system with connected resistive loads, such as a street lighting system.